= Optical gradient force
An optical gradient force arises from spatial variation in illumination. In a usual <optical trap> for a suitable transparent particle with higher <refractive index> than its surroundings, it draws the particle towards high <light intensity>. It supplies a <restoring force> near a stable trap, but three-dimensional confinement also depends on the <optical scattering force>. The direction and size depend on the particle and field; attraction to the brightest point is not universal. <Observation of a single-beam gradient force optical trap for dielectric particles by Arthur Ashkin and others>[Ashkin and colleagues, single-beam gradient optical trap, 1986]{ref}.
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